This invention generally relates to a system and method for determining the length of a discontinuity in an electrically conductive object using a probe which includes a plurality of eddy current coils which may be actuated independently. The system is specifically concerned with inspecting the interiors of the heat exchanger tubes of nuclear steam generators.
Systems for inspecting metallic objects for defects are well known in the art, particularly for tube wall inspection. Such systems typically employ a probe having one or more eddy current coils to detect the presence or absence of discontinuities in the tube wall created by cracks or pits. Generally, an alternating current is conducted through the coil or coils to emanate a time-varying magnetic field which in turn induces eddy currents in the inner walls of the tube as the coil is moved axially. Because the eddy currents create a magnetic field which is opposite in polarity to the time-varying magnetic field emanated by the probe coil, the eddy currents generated in the tube apply a measurable impedance to the alternating current that fluctuates through the coil. This impedance is highest when the metal conducting the eddy current is free from discontinuities such as cracks or pits or other imperfections of the metal.
Prior art eddy current probes typically have one or two coils which are moved along the longitudinal axis of the heat exchanger tube being inspected by a cable-pusher mechanism. When the eddy current coils come in the vicinity of a crack or a pit, or other discontinuity in the metal wall of the tube, the system operator takes note of the axial position of the probe at the time when the electromagnetic field emanated by the probe beings to interact with the discontinuity, as well as the point at which such interaction ceases. Because the system operator knows how many centimeters of flexible cable have been extended upwardly through the open end of the tube at the time when the flow is first detected and last detected by the eddy current probe coils, the axial position of the flaw along the longitudinal axis of the tube may be determined. Unfortunately, the drive cable used to axially extend and withdraw the probe along the interior of the tube has some amount of tensile and compressive yieldability, which in turn introduces inaccuracies in the axial measurements of the location of flaws along the longitudinal axis of the tube. Moreover, while such probes have proven themselves capable of generally locating the position of the flaw along the longitudinal axis of such tubes, the relatively rapid axial movement of the probe, in combination with the aforementioned tensile and compressive properties of the pusher cable, makes it particularly difficult for the system operator to accurately locate the extremities of the crack or other flaw. This is a significant limitation, as the axial length of such cracks or other flaws is an important factor in determining both the overall condition of the heat exchanger tube, as well as the appropriate maintenance operation (which may involve either sleeving the tube, or plugging it in cases of severe degradation).
Of course, a slower axial movement of the probe through the tube can enhance the accuracy of the system in determining the precise locations of the flaw extremities. A more positive driving mechanism, such as a lead screw, may also be used to both axially and rotatably sweep the probe around the interior of the tube walls in a helical fashion to obtain more detailed information concerning the location of the flaw extremities, as well as the orientation of the flaws. Such a lead screw driving mechanism has been used in conjunction with the pancake-type eddy current coil probes developed and patented by the Westinghouse Electric Corporation. However, while the slower and more precise driving of eddy current probes through such heat exchanger tubes does result in enhanced flaw detection accuracy, it does so at the expense of increased inspection times. This is a significant limitation, as such nuclear steam generators typically have over 40 miles of tubing, and as every day of down time caused by such tube inspections typically caused the electric utility over $500,000.00 a day in lost revenues.
Clearly, there is a need for a system and method for rapidly and accurately detecting the lengths of cracks and other discontinuities in the heat exchanger tubes of nuclear steam generators to determine the axial extent of damage that may exist therein so that the most appropriate maintenance procedures may be correctly chosen. Ideally, such an inspection system should allow determination of the length of a defect located in a tube without requiring the pushing and pulling of long drive cables which can axially stretch or compress, thereby limiting the accuracy of the measurement due to slack or "wind-up", or the use of relatively show helical drive mechanisms which provide the desired accuracy at the expense of increased operating time.